The primary purpose for checking crankshaft alignment is to determine if the main bearings are in exact or true alignment. If any distortion to the crankshaft alignment is present, it means either that there is excessive wear on one or more of the main bearings supporting the crankshaft or that the engine base is not being supported on a true horizontal plane. As a matter of practice, crankshaft alignment should be checked on a regular basis for the crankshafts in large diesel and gas engines, as well as in large compressors of the reciprocating type. Recommended intervals vary, but checking after 8,000 hours of use is recommended. Crankshaft alignment in general should be checked when the engine temperature is reasonably close to operating conditions and should always be checked after foundation bolts have been tightened or any adjustment is made to the rails or the sole plates for the engine. The two methods most commonly used to determine crankshaft alignment of an engine in service is by checking the crank web deflection at each throw or through the use of a bridge gage to determine the elevation of the crankshaft journal with relation to each main bearing support. The recommended procedure is that crankshaft alignment be determined by checking crank web deflection.
Crankshaft web deflection is most accurately measured with a deflection gage. In the past, any number of feeler gages have been utilized for this purpose, most notably the Starret Model 696 crankshaft distortion dial gage. In practice, a mechanic, upon stopping an engine, removes the side covers and either reaches into or climbs inside the crankcase. While the engine is still hot, usually on the order of 120.degree. Fahrenheit, a gage is placed between the walls of adjacent throws and the change in openings between the walls is observed as the crank is rotated something less than 360 degrees.
The temperature of the engine, the presence of hot oil, the close confines of the engine, and the difficulty of reading the gage in its extreme position, usually with a mirror, all operate to make web deflection measurements difficult and therefore suspect, particularly since a variation of only 0.001 inches is cause for concern. The job of making the web deflection measurement is usually relegated to the most junior person due to the difficult and time-consuming aspects of the measurement. If an inexperienced person were to misread the web deflection, depending upon the direction and magnitude of the error, a crankshaft failure or an unnecessary realignment process could result. More importantly, as the crankshaft rotates, the gage rotates relative to the webs in punch marks provided in opposing webs. Because the Starrett gages have dead centers, e.g., measuring pins which are fixed to the tool, gross measuring errors occur. Since the punch marks may not be symmetrical, the rotation of the corresponding pins in the punch marks results in large measuring errors. It will be appreciated that a punch mark is not designed to be a bearing, although it serves in this manner with Starrett gages. Thus the punch mark serves as a very imprecise bearing causing large measurement errors.
Mechanical or electromechanical feeler gages are illustrated by U.S. Pat. Nos. 3,119,187; 3,233,329; 3,958,337; 4,034,477; 4,087,918; 4,172,325; and 4,279,079. With respect to this last-mentioned patent, a direct feeling gage is provided in which pivoted feeler arms have two paced-apart transducer elements at the ends of the arms opposite the feeler ends. The spacing of the transducer elements is dependent upon the distance measured by the feeler arms. Referring to U.S. Pat. No. 3,958,337, a differential transformer is provided in a direct feeling measurement tool which includes a movable core serving as a coupling element for the differential transformer. The movable core is suspended by a wire which runs between a movable arch and a spring mounted to a fixed portion of the tool. The movement of the transformer core is in response to the flexing of the arch which is turn is caused by the relative spacing of two opposed feeler members. The movement of the feeler members is along a line perpendicular to the line along which the movable core moves. This device transforms movement in one direction to movement in another direction by virtue of the flexure of the arch which may be either U- or V-shaped. As a result, very large displacements of the feeler fingers result in a relatively small movement of the top of the arch. The accuracy of this device not only depends on the flexure characteristic of the arch, but also is diminished due to the relatively large amount of motion between the feeler fingers necessary to produce movement of the arch. In fact, the entire measuring system is determined by the flexible characteristics of the supports for the feeler fingers, as well as the flexure characteristic of the arch itself. This results in nonlinear measurements and measurements which are exceedingly error-prone.